Reply by hairy

The sides are mirror images of each other. I made a jig to do the routing. I used a piece of plywood with center lines marked on it. The blank had center lines marked on it. I centered the blank on top of the plywood jig by lining up the center lines. Then I screwed down some scrap wood around the blank to keep the blank from moving.I made stops on the outside perimeter of the plywood jig to limit how far the router could travel. I also made a 12” square router base to keep it flat at all times.

I centered the part of the blank that I wanted routed out, which is different from centering the blank on the jig. I did not use a bearing to follow a pattern, I restricted the travel of the router. In my case I needed a 4” square routed. The travel of the router was 2” out from the center.